Implantable cardioverter defibrillators (ICDs) are capable of detecting cardiac arrhythmias and delivering electrical stimulation therapies to terminate the detected arrhythmias. Tachycardia may be terminated by anti-tachycardia pacing therapies or high-voltage cardioversion shocks. Fibrillation may be terminated by high-voltage defibrillation shocks. These high-voltage shocks, which are referred to inclusively herein as “cardioversion/defibrillation shocks,” can be life-saving to a patient but can be very painful.
Atrial arrhythmias, such as atrial tachycardia (AT) and atrial fibrillation (AF), may not be directly life threatening and may occur repeatedly in some patients. Therefore, in order to avoid delivering frequent, painful shock therapies, atrial cardioversion/defibrillation therapies employing high-voltage shocks may be programmed to be disabled in an ICD or programmed to be delivered after the AT/AF episode has been detected for a sustained period of time, for example 2 hours or longer. Atrial arrhythmia detection algorithms may remain enabled because a physician may want to monitor for the presence of AT and AF for the purposes of managing medical therapies, such as anti-coagulation therapy and anti-arrhythmic drugs. Furthermore, non-painful, anti-tachycardia pacing therapies may be delivered in an attempt to terminate a detected atrial arrhythmia. If these less aggressive therapies fail, however, or if all atrial arrhythmia therapies are disabled, the atrial arrhythmia may be sustained for long periods of time.
A dual arrhythmia is the presence of ventricular fibrillation (VF) or ventricular tachycardia (VT) preceded by the onset of and co-existing with AF or AT. Retrospective analysis of arrhythmia incidence in patients implanted with the Medtronic Model 7250 dual chamber ICD revealed that atrial fibrillation (AF) is a co-existent arrhythmia with ventricular tachycardia (VT) or ventricular fibrillation (VF) in a significant patient population. Approximately 18% of all VF episodes and 3% of all VT episodes were accompanied by recent onset AF or AT. Stein KM et al., J Am Coll Cardiol Proc., 1999.
In modern ICDs, the delivery of cardioversion/defibrillation shocks can be programmed according to a number of delivery parameters such as the shock vector, the shock energy, shock waveform, shock pulse shape and the tilt. The electrodes selected from the implanted lead system associated with the ICD determine the shock vector. The tilt is the percentage by which the high-voltage output pulse decreases in amplitude before output is truncated. The shock waveform can be monophasic, biphasic, triphasic, etc., and the shock pulse shape may be ramped, square, etc.
Depending on these selected parameters, a cardioversion/defibrillation shock delivered to terminate VT/VF may also terminate AT or AF if present. There is some risk, however, in terminating AT or AF when the duration of the AT/AF episode and the anti-coagulation status of the patient are unknown. During sustained AT/AF episodes, blood stasis in the atria can result in the formation of clots or thrombus. If AT/AF is suddenly terminated, coordinated atrial contraction may dislodge the clot, producing thromboembolism and leading to a high risk of stroke. Conversely, conversion of recent onset AT/AF simultaneously with the ventricular arrhythmia treatment would be more desirable than converting just the ventricular fibrillation or tachycardia alone.
Currently, ICDs control arrhythmia therapy delivery based on the type of arrhythmia detected independent of the rhythm of the opposite chamber. Detection of ventricular arrhythmias generally takes precedence over the detection of atrial arrhythmias because of the more serious nature of ventricular arrhythmias. Reference is made, for example, to U.S. Pat. No. 5,545,186 issued to Olson et al., which generally discloses a prioritized rule-based algorithm for arrhythmia detection, incorporated herein by reference in its entirety. Such prioritized arrhythmia detection has important advantages in detecting and treating the most lethal forms of arrhythmias first. Once a ventricular arrhythmia has been diagnosed by review of both atrial and ventricular information, therapies are delivered for treatment of the ventricular arrhythmia independent of the atrial rhythm at the time.
From the above discussion, however, it is apparent that it is desirable to take into account the status of the atrial rhythm when selecting the configuration of ventricular cardioversion/defibrillation therapy. A need remains, therefore, for a method that allows the configuration of a cardioversion/defibrillation therapy delivered in response to ventricular arrhythmia detection to be selected based on the status of both the atrial and ventricular rhythms.